This invention relates generally to subsonic aircraft and relates in particularly to highly swept wing-tip mounted lifting surfaces to deter aircraft stall-spin entry.
Numerous solutions have been proposed by industry and government researchers over the years to eliminate or minimize aircraft stall and spin. It has been determined that stall-spin entry can be prevented by means of an active system or a passive system. An active system is intended to eliminate airplane stall and therefore airplane spin. Of the two types of active systems now employed, one is the active stall warning system which functions to warn the pilot of an impending stall and consequently permit him to take measures to prevent the airplane stall. This type of system includes stall horns, control-stick shakers, control-stick pushers and other warning devices. The other type of active system employed is the active stall deterrent system which automatically limits airplane angle of attack to one or two degrees less than the stall angle of attack to thereby prevent airplane stall and subsequent spin.
A large number of passive systems are available to prevent airplane stall-spin entry. Research at NASA Langley Research Center has determined that two passive systems are most successful in providing stall-spin protection for subsonic airplanes. The first system is the wing leading-edge droop modification. This modification has been developed for subsonic airplanes with no or modest degrees of wing sweep. Generally, when the leading-edge droop is added to the outboard portion of the wing, good airplane stall characteristics and spin resistance are obtained. The leading-edge droop modification consists of a glove over the forward part of the airfoil which provides a chord extension of approximately three percent and a droop which increases the leading-edge camber. The addition of a leading edge droop on the outboard portion of the wing delays stall of the outboard-wing portion to very high angles of attack and therefore deters airplane spin. The effectiveness of the wing leading-edge droop modification in delaying outboard-wing stall to high angles of attack is caused by vortex flow at the inboard end of the droop, which prevents separated flow from progressing outboard on the wing. The core of this vortex follows the direction of the freestream airflow.
The second passive system is the canard-configuration concept. A canard-airplane configuratin has a forward-mounted horizontal tail surface or canard surface and an aft-mounted wing. The canard surface is designed to have a lower stall angle of attack as compared to the wing stall angle of attack. Therefore, at high angles of attack the canard surface will stall prior to the wing. The lift contribution of the installed wing then dominates and produces a stabilizing nose-down pitching moment. As a result of this stabilizing pitching moment, the maximum angle of attack is limited to an angle well below the value required for wing stall. Thus, airplane stall and consequently, airplane spin are prevented.
Although each of the discussed prior art systems have their advantages they also have disadvantages. For example, although active stall warning systems warn the pilot of an approaching stall, they do not automatically prevent airplane stall and subsequent spin. Active stall warning systems also are more complicated than passive systems, and the potential failure modes of active systems are greater than passive systems. Further, as a result of the complexity and potential failure modes of active systems, more maintenance is required and active systems have generally higher weight than passive systems. The main disadvantage of the wing leading edge droop modification, a passive system, is that it tends to be ineffective when added to a wing having relatively blunt leading edge airfoil sections. Another disadvantage is the increase in cruise drag resulting from the increase in leading edge camber.
Although the canard configuration passive system effectively prevents airplane stall-spin entry, the required changes in airplane system layout and airplane aerodynamic design restrict the use thereof. It is not feasible to modify an airplane, from a conventional forward mounted wing configuration into a canard aft mounted wing configuratin, except in the very early stages of the airplane design. Also, the effectiveness of the stall resistance characteristics provided by the canard configuration concept can be influenced by many design variables, including relative geometry of the canard and wing, canard and wing airfoil sections, engine-propeller slipstream effects and center-of-gravity location. These variables can seriously affect the stall angle of attack of both canard and wing, and thereby negatively influence the stall resistance characteristics of the canard configuration.